Add PICO_FLASH_SIZE_BYTES to PIO (#1416)

Author: maxgerhardt

tools/platformio-build.py

@@ -126,7 +126,10 @@ def is_pio_build():
         ("BOARD_NAME", '\\"%s\\"' % env.subst("$BOARD")),
         "ARM_MATH_CM0_FAMILY",
         "ARM_MATH_CM0_PLUS",
-        "TARGET_RP2040"
+        "TARGET_RP2040",
+        # at this point, the main.py builder script hasn't updated upload.maximum_size yet,
+        # so it's the original value for the full flash.
+        ("PICO_FLASH_SIZE_BYTES", board.get("upload.maximum_size"))
     ],
 
     CPPPATH=[